In a telephone call system such as a landline telephone system or a mobile telephone system, usually bandlimited audio signals are transmitted or received. For the purpose of enhancing the sound quality, a technique is known that extends the bandwidth of bandlimited audio signals. For example, a technique is known where the folding of a digital signal is bandlimited with a low pass filter that is switched between a low cutoff frequency for a voiced interval and a high cutoff frequency for an unvoiced interval, thereby broadening the bandwidth to a higher frequency within the unvoiced interval. Another example is where a waveform of a sound source is generated from a narrow band signal, a low frequency signal obtained through a low pass filter whose cutoff frequency is the lowest frequency of a narrow band, a period of the narrow band signal, and the amplitude of the narrow band signal; and an audio signal having a broadband width is obtained by the summation of a high frequency signal obtained through a high pass filter and a high frequency component signal of an unvoiced sound. Further another example is where a fundamental frequency of a narrow band signal is extracted; a linear predictive residual is obtained from the linear predictive analysis of the narrow band signal; the linear predictive residual is shifted toward the frequency axis by the amount of an integer multiple of the fundamental frequency; a band-extended signal is obtained by the linear predictive synthesis; and a broadband audio signal is obtained by adding the narrow band signal and the band-extended signal.
For examples of the technologies above, refer to Japanese Laid-open Patent Publication Nos. 2002-82685, H9-258787, and H9-55778.
FIGS. 1 and 2 are diagrams depicting one example of a spectrum of an audio signal (spectrum of broadband sound) where a high frequency component has been ideally estimated from a low frequency component of a bandlimited audio signal. FIG. 1 depicts a spectrum of broadband sound when the fundamental frequency is high (345 Hz) and FIG. 2 depicts a case of a low fundamental frequency (125 Hz). The average of the fundamental frequency of a male voice is about 100 Hz and of a female voice 200 Hz or more. The inventors of the present invention have found a characteristic of broadband sound in that when the fundamental frequency is high, the difference of volumes (difference of power) between a high frequency region and a low frequency region is small and when the fundamental frequency is low, the difference of volumes is large (see FIGS. 1 and 2).
However, the conventional techniques do not consider the characteristic depicted in FIGS. 1 and 2. According to the conventional techniques, the high frequency component is generated in a single way irrespective of fundamental frequency. This causes a problem in that when the high frequency component having as large volume as the low frequency component is generated under a low fundamental frequency, the volume of the high frequency component becomes too large compared to an ideal volume and the sound quality is degraded. When the high frequency component has a smaller volume than the low frequency component under a high fundamental frequency, the volume of the high frequency component becomes too small compared to an ideal volume and cannot obtain sufficient band broadening effect. In other words, high quality sound cannot be produced.